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aplastic anemia bone marrow biopsy hypocellular

Histopathology: Bone marrow biopsy prepared for light microscopy shows diffuse infiltration by hairy cells rather than well-formed nodules. The marrow is often hypercellular in established disease but can be hypocellular in early stages, mimicking aplastic anemia. Hairy cells appear monotonous with oval nuclei and abundant clear cytoplasm that yields a characteristic fried egg morphology on H&E sections. In some cases, hair-like cytoplasmic projections (hairy indentations) are accentuated with specialized staining such as DBA.44 immunostain, supporting a hairy cell leukemia (HCL) diagnosis. The tumor cells may display spindled forms; the immunophenotype typically includes DBA.44 positivity, and cells may express CD11c, CD25, and CD103 on broader panels. Normal hematopoietic elements, particularly myeloid precursors, are reduced and can show dysplastic changes reminiscent of myelodysplastic syndrome. Plasma cells and mast cells may be mildly increased. Significant reticulin fibrosis is common, contributing to difficult marrow aspirates and making core biopsy essential to determine the extent of marrow replacement. Although marrow evaluation is informative, many cases of HCL can be diagnosed from peripheral blood smear and immunophenotyping alone; however, the biopsy provides baseline disease burden and a reference for assessing therapeutic response and fibrosis progression. This image exemplifies classic HCL marrow involvement with fried egg cells and conspicuous reticulin fibrosis.

Histopathology: Bone marrow biopsy prepared for light microscopy shows diffuse infiltration by hairy cells rather than well-formed nodules. The marrow is often hypercellular in established disease but can be hypocellular in early stages, mimicking aplastic anemia. Hairy cells appear monotonous with oval nuclei and abundant clear cytoplasm that yields a characteristic fried egg morphology on H&E sections. In some cases, hair-like cytoplasmic projections (hairy indentations) are accentuated with specialized staining such as DBA.44 immunostain, supporting a hairy cell leukemia (HCL) diagnosis. The tumor cells may display spindled forms; the immunophenotype typically includes DBA.44 positivity, and cells may express CD11c, CD25, and CD103 on broader panels. Normal hematopoietic elements, particularly myeloid precursors, are reduced and can show dysplastic changes reminiscent of myelodysplastic syndrome. Plasma cells and mast cells may be mildly increased. Significant reticulin fibrosis is common, contributing to difficult marrow aspirates and making core biopsy essential to determine the extent of marrow replacement. Although marrow evaluation is informative, many cases of HCL can be diagnosed from peripheral blood smear and immunophenotyping alone; however, the biopsy provides baseline disease burden and a reference for assessing therapeutic response and fibrosis progression. This image exemplifies classic HCL marrow involvement with fried egg cells and conspicuous reticulin fibrosis.

Light microscopy of a bone marrow biopsy from hairy cell leukemia (HCL) shows diffuse infiltration by small to medium lymphoid cells with abundant clear cytoplasm and oval, occasionally reniform nuclei; the cytoplasm creates a fried egg appearance. Hairy projections may be highlighted by immunostaining and by DBA.44 positivity, along with CD11c, CD25, and CD103 markers. The marrow is commonly hypercellular in established disease, whereas early involvement may be hypocellular and mimic aplastic anemia. Prominent reticulin fibrosis is typical and can hinder aspirate yield, making core biopsy essential for assessing infiltration and baseline disease burden. Normal hematopoietic elements, especially the myeloid lineage, are reduced and may display dysplastic changes resembling myelodysplastic syndrome. Plasma cells and mast cells may be mildly increased. In some cases, hairy cells assume a spindle-shaped morphology. The neoplastic cells are typically monotonous with minimal prominent nucleoli, and cytoplasm may exhibit cytoplasmic processes. Unlike several small B-cell lymphomas, discrete cellular aggregates are uncommon, with diffuse replacement predominating. Immunohistochemistry and flow cytometry confirm the diagnosis via characteristic immunophenotype. Clinically, marrow involvement informs staging, prognosis, and treatment planning, though peripheral blood smear and immunophenotypic profiling often suffice for diagnosis. This image, captured at high magnification on a hematoxylin and eosin stained section, showcases classic hairy cell features.

Light microscopy of a bone marrow biopsy from hairy cell leukemia (HCL) shows diffuse infiltration by small to medium lymphoid cells with abundant clear cytoplasm and oval, occasionally reniform nuclei; the cytoplasm creates a fried egg appearance. Hairy projections may be highlighted by immunostaining and by DBA.44 positivity, along with CD11c, CD25, and CD103 markers. The marrow is commonly hypercellular in established disease, whereas early involvement may be hypocellular and mimic aplastic anemia. Prominent reticulin fibrosis is typical and can hinder aspirate yield, making core biopsy essential for assessing infiltration and baseline disease burden. Normal hematopoietic elements, especially the myeloid lineage, are reduced and may display dysplastic changes resembling myelodysplastic syndrome. Plasma cells and mast cells may be mildly increased. In some cases, hairy cells assume a spindle-shaped morphology. The neoplastic cells are typically monotonous with minimal prominent nucleoli, and cytoplasm may exhibit cytoplasmic processes. Unlike several small B-cell lymphomas, discrete cellular aggregates are uncommon, with diffuse replacement predominating. Immunohistochemistry and flow cytometry confirm the diagnosis via characteristic immunophenotype. Clinically, marrow involvement informs staging, prognosis, and treatment planning, though peripheral blood smear and immunophenotypic profiling often suffice for diagnosis. This image, captured at high magnification on a hematoxylin and eosin stained section, showcases classic hairy cell features.

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Aplastic Anemia - Causes, Etiology, and Treatment

Definition

Aplastic anemia is pancytopenia with bone marrow hypocellularity - a condition where multipotent myeloid stem cells are suppressed, leading to bone marrow failure. The marrow is often virtually devoid of recognizable hematopoietic elements. It must be distinguished from pure red cell aplasia (only erythroid progenitors affected) and myelophthisic anemia (marrow infiltration by tumor/granulomas).
  • Harrison's Principles of Internal Medicine, 22nd Ed., p. 852
Pathophysiology of aplastic anemia - showing three etiologies and their treatments

Epidemiology

  • Incidence: 2 per million/year in Europe and Israel; 5-7 per million/year in Thailand and China
  • Affects both sexes equally
  • Bimodal age distribution: major peak in teens and twenties, second peak in older adults

Etiology (Causes)

Aplastic anemia is classified as acquired or constitutional (inherited):

Acquired Causes

CategoryExamples
RadiationMedical radiation therapy, accidental/occupational exposure; damages DNA in actively dividing cells
ChemicalsBenzene (most notorious); industrial solvents
Drugs - dose-dependentChemotherapy agents, conditioning regimens for transplant (all recipients affected predictably)
Drugs - idiosyncraticChloramphenicol, NSAIDs, sulfonamides, antiepileptics, gold - no clear dose-response; rare but unpredictable
Viral infectionsEBV (infectious mononucleosis), seronegative hepatitis (non-A, non-B, non-C), Parvovirus B19, HIV-1
Immune diseasesEosinophilic fasciitis, hypoimmunoglobulinemia, large granular lymphocytosis (LGL), thymoma/thymic carcinoma, graft-versus-host disease
PNHParoxysmal nocturnal hemoglobinuria (closely related; clonal overlap)
PregnancyRare association
Idiopathic (immune)Most cases - no identifiable trigger; immune-mediated destruction is presumed

Constitutional/Inherited (IBMFS - Inherited Bone Marrow Failure Syndromes)

SyndromeFeatures
Fanconi anemiaPhysical anomalies (radial defects, cafe-au-lait spots), childhood onset
Dyskeratosis congenitaTelomere biology disorder; nail dystrophy, oral leukoplakia, skin pigmentation
Shwachman-Diamond syndromeExocrine pancreatic insufficiency + marrow failure
GATA2 deficiencyCan present as marrow failure in normal-appearing adults
RUNX1 familial platelet disorderGermline predisposition
Diamond-Blackfan anemiaPure red cell aplasia variant
  • Harrison's, p. 852-853

Pathogenesis

Two major mechanisms are recognized (not mutually exclusive):

1. Immune-Mediated (Extrinsic) - Most Common

  • Stem cells are antigenically altered by drugs, infectious agents, or unknown environmental insults
  • This triggers a cellular immune response: activated Th1 cells produce cytokines (IFN-γ, TNF-α) that suppress hematopoietic progenitors
  • Evidence: immunosuppressive therapy directed against T cells restores hematopoiesis in 60-70% of patients

2. Intrinsic Stem Cell Abnormality

  • 5-10% of patients have inherited defects in telomerase → premature senescence of HSCs
  • An additional 50% of cases have unusually short telomeres (possibly from undiscovered telomerase defects or excessive HSC replication)
  • Genetically altered stem cells may also express neoantigens, making them targets for T-cell attack
  • Robbins & Kumar Basic Pathology, p. 443

Clinical Features

  • Pancytopenia causes three major symptom clusters:
    • Anemia: insidious weakness, pallor, dyspnea (slowly progressive)
    • Thrombocytopenia: petechiae, ecchymoses, mucosal bleeding
    • Neutropenia: recurrent serious infections
  • No splenomegaly - if present, another diagnosis should be sought
  • Diagnosis: bone marrow biopsy showing hypocellularity (>70% fat spaces), pancytopenia on CBC

Severity Classification (International AA Study Group)

SeverityCriteria
Severe (SAA)BM cellularity <25%, plus ≥2 of: neutrophils <0.5×10⁹/L, platelets <20×10⁹/L, reticulocytes <20×10⁹/L
Very Severe (vSAA)SAA criteria + neutrophils <0.2×10⁹/L
Moderate/Non-severeDoes not meet above thresholds

Treatment

1. Hematopoietic Stem Cell Transplantation (HSCT)

  • Treatment of choice for young patients (ideally <40 years) with severe/very severe AA who have a matched sibling donor
  • Often curative
  • Requires conditioning with immunosuppressive chemotherapy/radiation
  • Minimize pre-transplant transfusions - sensitization to alloantigens causes high engraftment failure rates

2. Immunosuppressive Therapy (IST)

  • For patients who are not transplant candidates (older age, no matched donor, or non-severe disease)
  • Standard regimen: Anti-thymocyte globulin (ATG) + Cyclosporine A (CsA)
    • Horse ATG (hATG) preferred over rabbit ATG based on clinical evidence
    • Restores hematopoiesis in 60-70% of patients
  • Eltrombopag (thrombopoietin receptor agonist) added to IST has significantly improved response rates - now standard in frontline treatment of SAA

3. Supportive Care

  • Red cell transfusions (minimize pre-transplant)
  • Platelet transfusions for bleeding risk
  • Hematopoietic growth factors (G-CSF, erythropoietin) - primarily for physical/chemical damage etiology
  • Antibiotics/antifungals for infection prophylaxis and treatment

4. Constitutional/Inherited Disease

  • Sex hormones (androgens like danazol) - for telomere biology disorders and Fanconi anemia
  • BMT - also applicable

Special Notes

  • If an inciting drug is identified, withdrawal sometimes leads to recovery (but rare)
  • Aplastic anemia has no splenomegaly - differentiates it from myelophthisic anemia and leukemia
  • Monitor for clonal evolution to MDS or PNH (especially after IST)

Summary Table

AspectKey Points
DefinitionPancytopenia + hypocellular bone marrow
Main causeImmune-mediated T-cell destruction of HSCs (idiopathic)
Key drugsChloramphenicol, NSAIDs, sulfonamides, chemotherapy
Key infectionsSeronegative hepatitis, EBV, parvovirus B19, HIV
DiagnosisBM biopsy (>70% fat), CBC pancytopenia
1st line if young + matched donorHSCT (curative)
1st line if no donor / olderhATG + Cyclosporine + Eltrombopag
No splenomegalyKey distinguishing feature
Sources: Harrison's Principles of Internal Medicine, 22nd Ed. | Robbins & Kumar Basic Pathology | PMID 38576005 | PMID 41569462
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